1. Field of the Invention
The present invention relates to techniques for mounting integrated circuits on circuit boards; and more particularly to a more efficient mounting assembly for multiple chip modules which allows easy assembly of computers using multiple chip modules.
2. Description of Related Art
A multiple chip module (MCM) consists of multiple unpackaged integrated circuits assembled into a sub-system the size of traditional single chip packages. One type of multiple chip module includes a substrate, a thin film interconnect structure, and a plurality of integrated circuits mounted on the interconnect structure, and encapsulated in an epoxy passivation material.
MCMs represent an important new trend in interconnect packaging, offering users higher performance, lower costs, and smaller form factors than traditional printed circuit boards with plated through hole and surface mount technologies. By densely mounting multiple unpackaged integrated circuits on to one substrate, distances between components are reduced, thereby frequently improving performance and typically reducing the form factor of the sub-system.
By eliminating the need for single chip packages, MCM technologies eliminate an entire level of interconnect, and permit significantly greater reductions in product footprint than other interconnect technologies. While the reduction in form factor is itself of great value, MCMs also tend to increase performance and reduce cost. By densely packaging multiple chips onto a substrate, distances are reduced, which generally increases performance because signals do not have to travel so far between components, and results in cost savings through higher yields and reduced material requirements.
Multiple chip modules also create unique mounting problems. In particular, dense functionality means that there are large number of input/output paths which must be connected from the multiple chip module to the circuit board on which they are mounted. Also, the density of functionality concentrates the sources of heat in a small module. Thus, multiple chip modules must be mounted in a manner which dissipates the heat generated in the small package.
Thus, multiple chip modules present a package with hundreds of leads in a very small footprint, and provide a concentrated source of heat which must be dissipated. For many applications, the heat must be dissipated without active cooling systems such as fans. Thus, the multiple chip module may often be mounted on a heat spreading system such a large heat conductive plate which spreads the heat over a large area.
Another problem facing integrated circuit mounting technologies is the ability to replace or upgrade existing chips. Thus, when a computer is sold with one generation host central processing unit, such as an Intel class 386 processor, and the user desires to upgrade the computer to a class 486 processor, the user must replace at least the entire mother board on the computer. Some special packaging technologies have been developed to support processor upgrades, but they involve increases in form factor for the mother board, and require specialized mounting technology.
Accordingly, it is desirable to implement a multiple chip module which provides for efficient thermal dissipation without use of a fan or other active cooling technology, reduces the size of the system over single chip component implementations, is low cost, provides good electrical characteristics which handle 100 MHz or higher clock rates, and is easily installed and replaced into a completely assembled laptop or notebook case.